The third wire is voltage sense from the plug which controls the regulator circuit in the ciggy plug body, if this wire breaks the voltage goes way high causing components to burn out in the navigation device (a known fact that has been occuring for some years).
If you are going to extend anything I would add a ciggy socket to the car on a length of cable and fit the unmodified charger lead in to that, bury the whole lot under the dash and you will have a decent install, have a look at
This thread for some ideasd - Mike
950, 940, 930, 720, One Third Edition, One XL, XL IQ Live and a dead 910 as well as PDA based software and an iFone 3G & 3GS